The present invention relates generally to impedance matching circuits used in phased array radar systems and more particularly to a transmit/receive (T/R) module which provides adaptive matching to the output impedance of a radar system's power amplifier during transmisson, and also to the input impedance of the low noise amplifier (LNA) during signal reception.
Phased array radar systems steer their beams electronically, and are generally preferable to the older mechanically-steered parabolic antennas. However, for different beam conditions, the impedance match required during transmit and receive is known to vary, and is not a constant value.
For optimum performance, the array antenna should be impedance matched with its transmitter during transmission, and with its receiver during reception. Ideally, if the array is impedance matched with the transmitter, then there is no reflection by the array during transmission, and all of the transmitter power will be radiated. Similarly, during reception the array should be impedance matched with the receiver.
The general design principles of impedance matching can be stated even more specifically. For maximum power transfer during transmission, the output impedance of the power amplifier should be the complex conjugate of the load. For minimum noise during reception the low noise amplifier's reflection coefficient should be matched with the reflection coefficient of the antenna.
A variety of systems are currently used in the art to accomplish the impedance matching objectives cited above. Two principal systems are: the fixed output matching impedance system; and the ferrite circulator.
The fixed output matching impedance system, as the title suggests, contains two impedance matching circuits which provide fixed impedance matching to the LNA and to the power amplifier. The disadvantage of this fixed impedance matching system is that it is limited, and capable of only fixed impedance matching However, the advantages of this system includes low cost, low weight, and comparatively small size.
Ferrite circulators provide variable and selectable impedance matching for power amplifiers and low noise amplifiers, and are commonly used in phased array radar systems. The advantage of ferrite circulators is that they are capable of much better impedance matching than the fixed output matching impedance systems. However, ferrite circulators are costly, bulky, and weigh much more than the fixed output matching impedance systems.
The task of providing an impedance matching system which combines the flexibility of ferrite circulators with lower cost, lower weight, and less bulk, is alleviated, to some degree, by the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 3,270,346 issued to Lerner; PA1 U.S. Pat. No. 3,794,941 issued to Templin; PA1 U.S. Pat. No. 4,311,972 issued to Landt et al; PA1 U.S. Pat. No. 4,351,060 issued to Treiber; and PA1 U.S. Pat. No. 4,502,025 issued to Carl, Jr. et al
Lerner is pertinent for its disclosure of selective connecting circuits for impedance matching an array of antennas. The patent discloses the matching of an array antenna to both transmitters and receivers. Polarization selective means such as the combination of transmission line and electric probes are used to provide a matching effect which varies with excitation condition so as to provide closer impedance matching for a number of excitation conditions, thereby improvrng the performance of the array antenna.
Carl, Jr., et al disclose an antenna coupler which acts as a solid state transmit/receive or TR switch for selectively coupling the antenna to either a transmitter or receiver. Templin shows an automatic antenna tuner including digital control circuits for selecting reactive tuning elements of an antenna impedance matching network.
In Landt et al an antenna coupler samples the power transferred between the transmitter and the load in response to the samples connects a tuning stub to the transmission lines to ensure matched impedance conditions between the transmitter and the load. A recursive digital filter is disclosed in Treiber for automatically digitally synthesizing an output impedance which matches a line.
The above-cited references are exemplary in the art and provide effective impedance matching systems which may serve as substitutes for the overly complex and bulky ferrite circulator or the simplistic fixed output impedance matching system. However, the need remains to combine the simplicity and reduced weight of the fixed output impedance matching system with the increased flexibility of the more complicated ferrite circulators and similar substitutes. The present invention is intended to satisfy that need.